1. Field of the Invention
The present invention is geared toward the field of integrated inductors, and particularly toward the construction of an integrated variable inductor whose inductance value may be adjusted on-chip.
2. Description of the Related Art
Inductors are an integral part of discrete radio frequency, RF, circuitry. Particularly, discrete variable inductors are typically used for tuning purposes.
With reference to FIG. 1, a discrete variable inductor 11 may consist of an inductor coil 12 between two end points (A1 and B1) and a mechanically adjustable armature 13 that may be slid along coil 12. One end 15 of armature 13 contacts coil 12 and the other end C1 provides an electrical contact for producing the desired variable inductance. The inductance value, L, of an inductor is dependent upon (among other factors) the number of windings in the coil between to electrical contact points, and one may adjust the number of winding between end points A1 and C1 by sliding armature 13 along coil 12. Such a variable inductor 11, however, is not available in integrated circuit technology, where mechanically adjustable armatures are not practical.
Nonetheless, fixed inductors are available in integrated circuits. With reference to FIG. 2, a typical integrated inductor 14 may be formed by a spiral of metal 12 on a first metal layer (such as metal layer M6) with two distinct endpoint connections A2 and B2, one on metal layer 6 and the other on a metal layer immediately below it, i.e. metal layer M5. A cross-section of inductor 14 fabricated in a typical silicon (Si) CMOS technology is shown in FIG. 2. A via 16 provides connection between spiral 12 on metal layer M6 and a metal lead 18 on metal layer M5, which provides the second endpoint B2. The entire inductor 14 is typically embedded in a dielectric 20 (such as SiO2) some finite distance above the Si substrate layer 22.
AC current flowing between endpoints A2 and B2 (through spiral 12 on metal layer M6, through via 16, and though lead 18 on metal layer M5) realizes the inductance. The characteristics of the inductor is primarily a function of the type of metal used (Al vs Cu), the number of spirals, the width of the metal lines, the spacing between the metal lines, and the distance between the metal layer M6 and the Si substrate 22. The overall value of inductance L is found by summing the self inductance of each wire segment and the positive and negative mutual inductance between all possible wire segment coupling pairs. The mutual inductance between two wires depends on their angle of intersection, length, and separation. Two wires orthogonal to each other have no mutual inductive coupling since their magnetic flux are not linked together. The current flow directions in the wires determine the sign of the coupling. The coupling is positive if the currents in the two wires are in the same direction and negative if the currents in the two wires are in the opposite direction. Process variations can alter any or all of physical characteristics that determine the inductor's inductance value.
An approach toward mitigating this variation between the designed, i.e. intended, operating point of an integrated inductor and its physical operating point after construction, is to construct a very large inductor whose process variations may be a small percentage of its intended value.
With reference FIG. 3, one such inductor may have multiple spirals on multiple process layers (i.e. metal layers) of an integrated circuit. For example, inductor 30 consists of three spirals 31, 32 and 33 on three respective metal layers M1, M2 and M3. Vias 35, 37 and 39 interconnect the three spirals 31, 32, and 33 to produce one large inductor with end points A3 and B3. FIG. 4 is an electrical representation of inductor 30 showing three inductor components 31-33 interconnected by vias 35-39 between end points A3 and B3.
Nonetheless, the large geographic area (which results in possible large amounts of capacitive coupling) as well as the many variations in their manufacturing process still make inductors highly susceptible to variations in their manufacturing process. As a result, the inductance value (and thereby the resonant operating point) of a physical integrated inductor is likely to differ from its intended operating point.
Nevertheless, on-chip inductors are still used pervasively in integrated RF circuit design applications. The impedance value of the inductor varies with frequency and determines the circuit's operating design point. In various circuit applications (e.g. voltage controlled oscillator, VCO), process parameter variations can alter the design point of a given circuit such that the optimized design value of an inductor turns out not to be an optimized value for the actual circuit application on-chip. Process variations can also alter the inductor's inductance value such that the circuit's physical operating point deviates from the desired design point. In such cases, one would like to modify the inductor impedance characteristics, and thereby the inductance value, on-chip by some noninvasive means. The ability to modify an inductor value on-chip would provide circuit designers a great deal of freedom to optimize their respective circuits post-process.